/*
    * Copyright 2013 The Netty Project
    *
    * The Netty Project licenses this file to you under the Apache License,
    * version 2.0 (the "License"); you may not use this file except in compliance
    * with the License. You may obtain a copy of the License at:
    *
    *   https://www.apache.org/licenses/LICENSE-2.0
    *
   * Unless required by applicable law or agreed to in writing, software
   * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
   * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
   * License for the specific language governing permissions and limitations
   * under the License.
   */
  package io.netty.util;
  
  import io.netty.util.concurrent.FastThreadLocal;
  import io.netty.util.concurrent.FastThreadLocalThread;
  import io.netty.util.internal.ObjectPool;
  import io.netty.util.internal.PlatformDependent;
  import io.netty.util.internal.SystemPropertyUtil;
  import io.netty.util.internal.UnstableApi;
  import io.netty.util.internal.logging.InternalLogger;
  import io.netty.util.internal.logging.InternalLoggerFactory;
  import org.jctools.queues.MessagePassingQueue;
  import org.jetbrains.annotations.VisibleForTesting;
  
  import java.util.ArrayDeque;
  import java.util.Objects;
  import java.util.Queue;
  import java.util.concurrent.atomic.AtomicIntegerFieldUpdater;
  
  import static io.netty.util.internal.PlatformDependent.newFixedMpmcQueue;
  import static io.netty.util.internal.PlatformDependent.newMpscQueue;
  import static java.lang.Math.max;
  import static java.lang.Math.min;
  
  /**
   * Light-weight object pool based on a thread-local stack.
   *
   * @param <T> the type of the pooled object
   */
  public abstract class Recycler<T> {
      private static final InternalLogger logger = InternalLoggerFactory.getInstance(Recycler.class);
  
      /**
       * We created this handle to avoid having more than 2 concrete implementations of {@link EnhancedHandle}
       * i.e. NOOP_HANDLE, {@link DefaultHandle} and the one used in the LocalPool.
       */
      private static final class LocalPoolHandle<T> extends EnhancedHandle<T> {
          private final UnguardedLocalPool<T> pool;
  
          private LocalPoolHandle(UnguardedLocalPool<T> pool) {
              this.pool = pool;
          }
  
          @Override
          public void recycle(T object) {
              UnguardedLocalPool<T> pool = this.pool;
              if (pool != null) {
                  pool.release(object);
              }
          }
  
          @Override
          public void unguardedRecycle(final Object object) {
              UnguardedLocalPool<T> pool = this.pool;
              if (pool != null) {
                  pool.release((T) object);
              }
          }
      }
  
      private static final EnhancedHandle<?> NOOP_HANDLE = new LocalPoolHandle<>(null);
      private static final UnguardedLocalPool<?> NOOP_LOCAL_POOL = new UnguardedLocalPool<>(0);
      private static final int DEFAULT_INITIAL_MAX_CAPACITY_PER_THREAD = 4 * 1024; // Use 4k instances as default.
      private static final int DEFAULT_MAX_CAPACITY_PER_THREAD;
      private static final int RATIO;
      private static final int DEFAULT_QUEUE_CHUNK_SIZE_PER_THREAD;
      private static final boolean BLOCKING_POOL;
      private static final boolean BATCH_FAST_TL_ONLY;
  
      static {
          // In the future, we might have different maxCapacity for different object types.
          // e.g. io.netty.recycler.maxCapacity.writeTask
          //      io.netty.recycler.maxCapacity.outboundBuffer
          int maxCapacityPerThread = SystemPropertyUtil.getInt("io.netty.recycler.maxCapacityPerThread",
                  SystemPropertyUtil.getInt("io.netty.recycler.maxCapacity", DEFAULT_INITIAL_MAX_CAPACITY_PER_THREAD));
          if (maxCapacityPerThread < 0) {
              maxCapacityPerThread = DEFAULT_INITIAL_MAX_CAPACITY_PER_THREAD;
          }
  
          DEFAULT_MAX_CAPACITY_PER_THREAD = maxCapacityPerThread;
          DEFAULT_QUEUE_CHUNK_SIZE_PER_THREAD = SystemPropertyUtil.getInt("io.netty.recycler.chunkSize", 32);
  
          // By default, we allow one push to a Recycler for each 8th try on handles that were never recycled before.
          // This should help to slowly increase the capacity of the recycler while not be too sensitive to allocation
          // bursts.
         RATIO = max(0, SystemPropertyUtil.getInt("io.netty.recycler.ratio", 8));
 
         BLOCKING_POOL = SystemPropertyUtil.getBoolean("io.netty.recycler.blocking", false);
         BATCH_FAST_TL_ONLY = SystemPropertyUtil.getBoolean("io.netty.recycler.batchFastThreadLocalOnly", true);
 
         if (logger.isDebugEnabled()) {
             if (DEFAULT_MAX_CAPACITY_PER_THREAD == 0) {
                 logger.debug("-Dio.netty.recycler.maxCapacityPerThread: disabled");
                 logger.debug("-Dio.netty.recycler.ratio: disabled");
                 logger.debug("-Dio.netty.recycler.chunkSize: disabled");
                 logger.debug("-Dio.netty.recycler.blocking: disabled");
                 logger.debug("-Dio.netty.recycler.batchFastThreadLocalOnly: disabled");
             } else {
                 logger.debug("-Dio.netty.recycler.maxCapacityPerThread: {}", DEFAULT_MAX_CAPACITY_PER_THREAD);
                 logger.debug("-Dio.netty.recycler.ratio: {}", RATIO);
                 logger.debug("-Dio.netty.recycler.chunkSize: {}", DEFAULT_QUEUE_CHUNK_SIZE_PER_THREAD);
                 logger.debug("-Dio.netty.recycler.blocking: {}", BLOCKING_POOL);
                 logger.debug("-Dio.netty.recycler.batchFastThreadLocalOnly: {}", BATCH_FAST_TL_ONLY);
             }
         }
     }
 
     private final LocalPool<?, T> localPool;
     private final FastThreadLocal<LocalPool<?, T>> threadLocalPool;
 
     /**
      * USE IT CAREFULLY!<br>
      * This is creating a shareable {@link Recycler} which {@code get()} can be called concurrently from different
      * {@link Thread}s.<br>
      * Usually {@link Recycler}s uses some form of thread-local storage, but this constructor is disabling it
      * and using a single pool of instances instead, sized as {@code maxCapacity}<br>
      * This is NOT enforcing pooled instances states to be validated if {@code unguarded = true}:
      * it means that {@link Handle#recycle(Object)} is not checking that {@code object} is the same which was
      * recycled and assume no other recycling happens concurrently
      * (similar to what {@link EnhancedHandle#unguardedRecycle(Object)} does).<br>
      */
     protected Recycler(int maxCapacity, boolean unguarded) {
         if (maxCapacity <= 0) {
             maxCapacity = 0;
         } else {
             maxCapacity = max(4, maxCapacity);
         }
         threadLocalPool = null;
         if (maxCapacity == 0) {
             localPool = (LocalPool<?, T>) NOOP_LOCAL_POOL;
         } else {
             localPool = unguarded? new UnguardedLocalPool<>(maxCapacity) : new GuardedLocalPool<>(maxCapacity);
         }
     }
 
     /**
      * USE IT CAREFULLY!<br>
      * This is NOT enforcing pooled instances states to be validated if {@code unguarded = true}:
      * it means that {@link Handle#recycle(Object)} is not checking that {@code object} is the same which was
      * recycled and assume no other recycling happens concurrently
      * (similar to what {@link EnhancedHandle#unguardedRecycle(Object)} does).<br>
      */
     protected Recycler(boolean unguarded) {
         this(DEFAULT_MAX_CAPACITY_PER_THREAD, RATIO, DEFAULT_QUEUE_CHUNK_SIZE_PER_THREAD, unguarded);
     }
 
     /**
      * USE IT CAREFULLY!<br>
      * This is NOT enforcing pooled instances states to be validated if {@code unguarded = true} as stated by
      * {@link #Recycler(boolean)} and allows to pin the recycler to a specific {@link Thread}, if {@code owner}
      * is not {@code null}.
      * <p>
      * Since this method has been introduced for performance-sensitive cases it doesn't validate if {@link #get()} is
      * called from the {@code owner} {@link Thread}: it assumes {@link #get()} to never happen concurrently.
      * <p>
      */
     protected Recycler(Thread owner, boolean unguarded) {
         this(DEFAULT_MAX_CAPACITY_PER_THREAD, RATIO, DEFAULT_QUEUE_CHUNK_SIZE_PER_THREAD, owner, unguarded);
     }
 
     protected Recycler(int maxCapacityPerThread) {
         this(maxCapacityPerThread, RATIO, DEFAULT_QUEUE_CHUNK_SIZE_PER_THREAD);
     }
 
     protected Recycler() {
         this(DEFAULT_MAX_CAPACITY_PER_THREAD);
     }
 
     /**
      * USE IT CAREFULLY!<br>
      * This is NOT enforcing pooled instances states to be validated if {@code unguarded = true} as stated by
      * {@link #Recycler(boolean)}, but it allows to tune the chunk size used for local pooling.
      */
     protected Recycler(int chunksSize, int maxCapacityPerThread, boolean unguarded) {
         this(maxCapacityPerThread, RATIO, chunksSize, unguarded);
     }
 
     /**
      * USE IT CAREFULLY!<br>
      * This is NOT enforcing pooled instances states to be validated if {@code unguarded = true} and allows pinning
      * the recycler to a specific {@link Thread}, as stated by {@link #Recycler(Thread, boolean)}.<br>
      * It also allows tuning the chunk size used for local pooling and the max capacity per thread.
      *
      * @throws IllegalArgumentException if {@code owner} is {@code null}.
      */
     protected Recycler(int chunkSize, int maxCapacityPerThread, Thread owner, boolean unguarded) {
         this(maxCapacityPerThread, RATIO, chunkSize, owner, unguarded);
     }
 
     /**
      * @deprecated Use one of the following instead:
      * {@link #Recycler()}, {@link #Recycler(int)}, {@link #Recycler(int, int, int)}.
      */
     @Deprecated
     @SuppressWarnings("unused") // Parameters we can't remove due to compatibility.
     protected Recycler(int maxCapacityPerThread, int maxSharedCapacityFactor) {
         this(maxCapacityPerThread, RATIO, DEFAULT_QUEUE_CHUNK_SIZE_PER_THREAD);
     }
 
     /**
      * @deprecated Use one of the following instead:
      * {@link #Recycler()}, {@link #Recycler(int)}, {@link #Recycler(int, int, int)}.
      */
     @Deprecated
     @SuppressWarnings("unused") // Parameters we can't remove due to compatibility.
     protected Recycler(int maxCapacityPerThread, int maxSharedCapacityFactor,
                        int ratio, int maxDelayedQueuesPerThread) {
         this(maxCapacityPerThread, ratio, DEFAULT_QUEUE_CHUNK_SIZE_PER_THREAD);
     }
 
     /**
      * @deprecated Use one of the following instead:
      * {@link #Recycler()}, {@link #Recycler(int)}, {@link #Recycler(int, int, int)}.
      */
     @Deprecated
     @SuppressWarnings("unused") // Parameters we can't remove due to compatibility.
     protected Recycler(int maxCapacityPerThread, int maxSharedCapacityFactor,
                        int ratio, int maxDelayedQueuesPerThread, int delayedQueueRatio) {
         this(maxCapacityPerThread, ratio, DEFAULT_QUEUE_CHUNK_SIZE_PER_THREAD);
     }
 
     protected Recycler(int maxCapacityPerThread, int interval, int chunkSize) {
         this(maxCapacityPerThread, interval, chunkSize, true, null, false);
     }
 
     /**
      * USE IT CAREFULLY!<br>
      * This is NOT enforcing pooled instances states to be validated if {@code unguarded =true}
      * as stated by {@link #Recycler(boolean)}.
      */
     protected Recycler(int maxCapacityPerThread, int interval, int chunkSize, boolean unguarded) {
         this(maxCapacityPerThread, interval, chunkSize, true, null, unguarded);
     }
 
     /**
      * USE IT CAREFULLY!<br>
      * This is NOT enforcing pooled instances states to be validated if {@code unguarded =true}
      * as stated by {@link #Recycler(boolean)}.
      */
     protected Recycler(int maxCapacityPerThread, int interval, int chunkSize, Thread owner, boolean unguarded) {
         this(maxCapacityPerThread, interval, chunkSize, false, owner, unguarded);
     }
 
     @SuppressWarnings("unchecked")
     private Recycler(int maxCapacityPerThread, int ratio, int chunkSize, boolean useThreadLocalStorage,
                      Thread owner, boolean unguarded) {
         final int interval = max(0, ratio);
         if (maxCapacityPerThread <= 0) {
             maxCapacityPerThread = 0;
             chunkSize = 0;
         } else {
             maxCapacityPerThread = max(4, maxCapacityPerThread);
             chunkSize = max(2, min(chunkSize, maxCapacityPerThread >> 1));
         }
         if (maxCapacityPerThread > 0 && useThreadLocalStorage) {
             final int finalMaxCapacityPerThread = maxCapacityPerThread;
             final int finalChunkSize = chunkSize;
             threadLocalPool = new FastThreadLocal<LocalPool<?, T>>() {
                 @Override
                 protected LocalPool<?, T> initialValue() {
                     return unguarded? new UnguardedLocalPool<>(finalMaxCapacityPerThread, interval, finalChunkSize) :
                             new GuardedLocalPool<>(finalMaxCapacityPerThread, interval, finalChunkSize);
                 }
 
                 @Override
                 protected void onRemoval(LocalPool<?, T> value) throws Exception {
                     super.onRemoval(value);
                     MessagePassingQueue<?> handles = value.pooledHandles;
                     value.pooledHandles = null;
                     value.owner = null;
                     if (handles != null) {
                         handles.clear();
                     }
                 }
             };
             localPool = null;
         } else {
             threadLocalPool = null;
             if (maxCapacityPerThread == 0) {
                 localPool = (LocalPool<?, T>) NOOP_LOCAL_POOL;
             } else {
                 Objects.requireNonNull(owner, "owner");
                 localPool = unguarded? new UnguardedLocalPool<>(owner, maxCapacityPerThread, interval, chunkSize) :
                         new GuardedLocalPool<>(owner, maxCapacityPerThread, interval, chunkSize);
             }
         }
     }
 
     @SuppressWarnings("unchecked")
     public final T get() {
         if (localPool != null) {
             return localPool.getWith(this);
         } else {
             if (!FastThreadLocalThread.currentThreadWillCleanupFastThreadLocals()) {
                 return newObject((Handle<T>) NOOP_HANDLE);
             }
             return threadLocalPool.get().getWith(this);
         }
     }
 
     /**
      * Disassociates the {@link Recycler} from the current {@link Thread} if it was pinned,
      * see {@link #Recycler(Thread, boolean)}.
      * <p>
      * Be aware that this method is not thread-safe: it's necessary to allow a {@link Thread} to
      * be garbage collected even if {@link Handle}s are still referenced by other objects.
      * <p>
      */
     public static void unpinOwner(Recycler<?> recycler) {
         if (recycler.localPool != null) {
             recycler.localPool.owner = null;
         }
     }
 
     /**
      * @deprecated use {@link Handle#recycle(Object)}.
      */
     @Deprecated
     public final boolean recycle(T o, Handle<T> handle) {
         if (handle == NOOP_HANDLE) {
             return false;
         }
 
         handle.recycle(o);
         return true;
     }
 
     @VisibleForTesting
     final int threadLocalSize() {
         if (localPool != null) {
             return localPool.size();
         } else {
             if (!FastThreadLocalThread.currentThreadWillCleanupFastThreadLocals()) {
                 return 0;
             }
             final LocalPool<?, T> pool = threadLocalPool.getIfExists();
             if (pool == null) {
                 return 0;
             }
             return pool.size();
         }
     }
 
     /**
      * @param handle can NOT be null.
      */
     protected abstract T newObject(Handle<T> handle);
 
     @SuppressWarnings("ClassNameSameAsAncestorName") // Can't change this due to compatibility.
     public interface Handle<T> extends ObjectPool.Handle<T>  { }
 
     @UnstableApi
     public abstract static class EnhancedHandle<T> implements Handle<T> {
 
         public abstract void unguardedRecycle(Object object);
 
         private EnhancedHandle() {
         }
     }
 
     private static final class DefaultHandle<T> extends EnhancedHandle<T> {
         private static final int STATE_CLAIMED = 0;
         private static final int STATE_AVAILABLE = 1;
         private static final AtomicIntegerFieldUpdater<DefaultHandle<?>> STATE_UPDATER;
         static {
             AtomicIntegerFieldUpdater<?> updater = AtomicIntegerFieldUpdater.newUpdater(DefaultHandle.class, "state");
             //noinspection unchecked
             STATE_UPDATER = (AtomicIntegerFieldUpdater<DefaultHandle<?>>) updater;
         }
 
         private volatile int state; // State is initialised to STATE_CLAIMED (aka. 0) so they can be released.
         private final GuardedLocalPool<T> localPool;
         private T value;
 
         DefaultHandle(GuardedLocalPool<T> localPool) {
             this.localPool = localPool;
         }
 
         @Override
         public void recycle(Object object) {
             if (object != value) {
                 throw new IllegalArgumentException("object does not belong to handle");
             }
             toAvailable();
             localPool.release(this);
         }
 
         @Override
         public void unguardedRecycle(Object object) {
             if (object != value) {
                 throw new IllegalArgumentException("object does not belong to handle");
             }
             unguardedToAvailable();
             localPool.release(this);
         }
 
         T claim() {
             assert state == STATE_AVAILABLE;
             STATE_UPDATER.lazySet(this, STATE_CLAIMED);
             return value;
         }
 
         void set(T value) {
             this.value = value;
         }
 
         private void toAvailable() {
             int prev = STATE_UPDATER.getAndSet(this, STATE_AVAILABLE);
             if (prev == STATE_AVAILABLE) {
                 throw new IllegalStateException("Object has been recycled already.");
             }
         }
 
         private void unguardedToAvailable() {
             int prev = state;
             if (prev == STATE_AVAILABLE) {
                 throw new IllegalStateException("Object has been recycled already.");
             }
             STATE_UPDATER.lazySet(this, STATE_AVAILABLE);
         }
     }
 
     private static final class GuardedLocalPool<T> extends LocalPool<DefaultHandle<T>, T> {
 
         GuardedLocalPool(int maxCapacity) {
             super(maxCapacity);
         }
 
         GuardedLocalPool(Thread owner, int maxCapacity, int ratioInterval, int chunkSize) {
             super(owner, maxCapacity, ratioInterval, chunkSize);
         }
 
         GuardedLocalPool(int maxCapacity, int ratioInterval, int chunkSize) {
             super(maxCapacity, ratioInterval, chunkSize);
         }
 
         @Override
         public T getWith(Recycler<T> recycler) {
             DefaultHandle<T> handle = acquire();
             T obj;
             if (handle == null) {
                 handle = canAllocatePooled()? new DefaultHandle<>(this) : null;
                 if (handle != null) {
                     obj = recycler.newObject(handle);
                     handle.set(obj);
                 } else {
                     obj = recycler.newObject((Handle<T>) NOOP_HANDLE);
                 }
             } else {
                 obj = handle.claim();
             }
             return obj;
         }
     }
 
     private static final class UnguardedLocalPool<T> extends LocalPool<T, T> {
         private final EnhancedHandle<T> handle;
 
         UnguardedLocalPool(int maxCapacity) {
             super(maxCapacity);
             handle = maxCapacity == 0? null : new LocalPoolHandle<>(this);
         }
 
         UnguardedLocalPool(Thread owner, int maxCapacity, int ratioInterval, int chunkSize) {
             super(owner, maxCapacity, ratioInterval, chunkSize);
             handle = new LocalPoolHandle<>(this);
         }
 
         UnguardedLocalPool(int maxCapacity, int ratioInterval, int chunkSize) {
             super(maxCapacity, ratioInterval, chunkSize);
             handle = new LocalPoolHandle<>(this);
         }
 
         @Override
         public T getWith(Recycler<T> recycler) {
             T obj = acquire();
             if (obj == null) {
                 obj = recycler.newObject(canAllocatePooled()? handle : (Handle<T>) NOOP_HANDLE);
             }
             return obj;
         }
     }
 
     private abstract static class LocalPool<H, T> {
         private final int ratioInterval;
         private final H[] batch;
         private int batchSize;
         private Thread owner;
         private MessagePassingQueue<H> pooledHandles;
         private int ratioCounter;
 
         LocalPool(int maxCapacity) {
             // if there's no capacity, we need to never allocate pooled objects.
             // if there's capacity, because there is a shared pool, we always pool them, since we cannot trust the
             // thread unsafe ratio counter.
             this.ratioInterval = maxCapacity == 0? -1 : 0;
             this.owner = null;
             batch = null;
             batchSize = 0;
             pooledHandles = createExternalMcPool(maxCapacity);
             ratioCounter = 0;
         }
 
         @SuppressWarnings("unchecked")
         LocalPool(Thread owner, int maxCapacity, int ratioInterval, int chunkSize) {
             this.ratioInterval = ratioInterval;
             this.owner = owner;
             batch = owner != null? (H[]) new Object[chunkSize] : null;
             batchSize = 0;
             pooledHandles = createExternalScPool(chunkSize, maxCapacity);
             ratioCounter = ratioInterval; // Start at interval so the first one will be recycled.
         }
 
         private static <H> MessagePassingQueue<H> createExternalMcPool(int maxCapacity) {
             if (maxCapacity == 0) {
                 return null;
             }
             if (BLOCKING_POOL) {
                 return new BlockingMessageQueue<>(maxCapacity);
             }
             return (MessagePassingQueue<H>) newFixedMpmcQueue(maxCapacity);
         }
 
         private static <H> MessagePassingQueue<H> createExternalScPool(int chunkSize, int maxCapacity) {
             if (maxCapacity == 0) {
                 return null;
             }
             if (BLOCKING_POOL) {
                 return new BlockingMessageQueue<>(maxCapacity);
             }
             return (MessagePassingQueue<H>) newMpscQueue(chunkSize, maxCapacity);
         }
 
         LocalPool(int maxCapacity, int ratioInterval, int chunkSize) {
             this(!BATCH_FAST_TL_ONLY || FastThreadLocalThread.currentThreadWillCleanupFastThreadLocals()
                          ? Thread.currentThread() : null, maxCapacity, ratioInterval, chunkSize);
         }
 
         protected final H acquire() {
             int size = batchSize;
             if (size == 0) {
                 // it's ok to be racy; at worst we reuse something that won't return back to the pool
                 final MessagePassingQueue<H> handles = pooledHandles;
                 if (handles == null) {
                     return null;
                 }
                 return handles.relaxedPoll();
             }
             int top = size - 1;
             final H h = batch[top];
             batchSize = top;
             batch[top] = null;
             return h;
         }
 
         protected final void release(H handle) {
             Thread owner = this.owner;
             if (owner != null && Thread.currentThread() == owner && batchSize < batch.length) {
                 batch[batchSize] = handle;
                 batchSize++;
             } else if (owner != null && isTerminated(owner)) {
                 pooledHandles = null;
                 this.owner = null;
             } else {
                 MessagePassingQueue<H> handles = pooledHandles;
                 if (handles != null) {
                     handles.relaxedOffer(handle);
                 }
             }
         }
 
         private static boolean isTerminated(Thread owner) {
             // Do not use `Thread.getState()` in J9 JVM because it's known to have a performance issue.
             // See: https://github.com/netty/netty/issues/13347#issuecomment-1518537895
             return PlatformDependent.isJ9Jvm()? !owner.isAlive() : owner.getState() == Thread.State.TERMINATED;
         }
 
         boolean canAllocatePooled() {
             if (ratioInterval < 0) {
                 return false;
             }
             if (ratioInterval == 0) {
                 return true;
             }
             if (++ratioCounter >= ratioInterval) {
                 ratioCounter = 0;
                 return true;
             }
             return false;
         }
 
         abstract T getWith(Recycler<T> recycler);
 
         int size() {
             MessagePassingQueue<H> handles = pooledHandles;
             final int externalSize = handles != null? handles.size() : 0;
             return externalSize + (batch != null? batchSize : 0);
         }
     }
 
     /**
      * This is an implementation of {@link MessagePassingQueue}, similar to what might be returned from
      * {@link PlatformDependent#newMpscQueue(int)}, but intended to be used for debugging purpose.
      * The implementation relies on synchronised monitor locks for thread-safety.
      * The {@code fill} bulk operation is not supported by this implementation.
      */
     private static final class BlockingMessageQueue<T> implements MessagePassingQueue<T> {
         private final Queue<T> deque;
         private final int maxCapacity;
 
         BlockingMessageQueue(int maxCapacity) {
             this.maxCapacity = maxCapacity;
             // This message passing queue is backed by an ArrayDeque instance,
             // made thread-safe by synchronising on `this` BlockingMessageQueue instance.
             // Why ArrayDeque?
             // We use ArrayDeque instead of LinkedList or LinkedBlockingQueue because it's more space efficient.
             // We use ArrayDeque instead of ArrayList because we need the queue APIs.
             // We use ArrayDeque instead of ConcurrentLinkedQueue because CLQ is unbounded and has O(n) size().
             // We use ArrayDeque instead of ArrayBlockingQueue because ABQ allocates its max capacity up-front,
             // and these queues will usually have large capacities, in potentially great numbers (one per thread),
             // but often only have comparatively few items in them.
             deque = new ArrayDeque<T>();
         }
 
         @Override
         public synchronized boolean offer(T e) {
             if (deque.size() == maxCapacity) {
                 return false;
             }
             return deque.offer(e);
         }
 
         @Override
         public synchronized T poll() {
             return deque.poll();
         }
 
         @Override
         public synchronized T peek() {
             return deque.peek();
         }
 
         @Override
         public synchronized int size() {
             return deque.size();
         }
 
         @Override
         public synchronized void clear() {
             deque.clear();
         }
 
         @Override
         public synchronized boolean isEmpty() {
             return deque.isEmpty();
         }
 
         @Override
         public int capacity() {
             return maxCapacity;
         }
 
         @Override
         public boolean relaxedOffer(T e) {
             return offer(e);
         }
 
         @Override
         public T relaxedPoll() {
             return poll();
         }
 
         @Override
         public T relaxedPeek() {
             return peek();
         }
 
         @Override
         public int drain(Consumer<T> c, int limit) {
             T obj;
             int i = 0;
             for (; i < limit && (obj = poll()) != null; i++) {
                 c.accept(obj);
             }
             return i;
         }
 
         @Override
         public int fill(Supplier<T> s, int limit) {
             throw new UnsupportedOperationException();
         }
 
         @Override
         public int drain(Consumer<T> c) {
             throw new UnsupportedOperationException();
         }
 
         @Override
         public int fill(Supplier<T> s) {
             throw new UnsupportedOperationException();
         }
 
         @Override
         public void drain(Consumer<T> c, WaitStrategy wait, ExitCondition exit) {
             throw new UnsupportedOperationException();
         }
 
         @Override
         public void fill(Supplier<T> s, WaitStrategy wait, ExitCondition exit) {
             throw new UnsupportedOperationException();
         }
     }
 }